Tutorial :dynamically loaded object loaded into a C program gives undefined symbol errors on x86_64



Question:

I have a C program that dynamically loads a .so file at runtime in order to connect to a MySQL database. On an x86 (32bit) kernel this works fine but when I recompile my program on an x86_64 (64 bit) kernel I get runtime errors like this:

dlerror:   mysql-1.932-x86_64-freebsd7.2.so::plugin_tweak_products: Undefined symbol "plugin_filter_cart"  dlerror:   mysql-1.932-x86_64-freebsd7.2.so::plugin_shutdown: Undefined symbol "plugin_post_action"  

Obviously from the error message above you can see that this program is running on a FreeBSD 7.2 x86_64 machine. Both the C program and the .so file are compiled for 64 bit.

I am passing RTLD_LAZY to dlopen() when I load the .so file. I think the problem is that for some reason on x86_64 it is not dynamically loading parts of the library as needed but on 32 bit x86 it is. Is there some flag I can put in my Makefile.am to get this to work on x86_64? Any other ideas?

Here is what the file command lists for my C program

ELF 64-bit LSB executable, x86-64, version 1 (FreeBSD), for FreeBSD 7.2, dynamically linked (uses shared libs), FreeBSD-style, not stripped  

and for the .so file

ELF 64-bit LSB shared object, x86-64, version 1 (FreeBSD), not stripped  


Solution:1

Just a wild guess. The prefix plugin seems to indicate there might be some callbacks with function pointers going on. Also probably your compiler versions are not the same for 32 and 64 bit? Do you use C99's or gcc's inline feature?

Such things can happen if one variant of your compiler is able to inline some function (static or inline) and the other doesn't. Then an external symbol might be produced or not. This depends a lot of your compiler version, gcc had different strategies to handle such situations over time. Try to enforce the implementation of the function in at least one of your objects. And as roguenut indicates, check with nm for the missing symbols.


Solution:2

It looks like this was being caused by the same problem as

dlerror: Undefined symbol "_nss_cache_cycle_prevention_function" on FreeBSD 7.2

You need to call dlerror() first and ignore the return value to clear out errors from previous errors before you check the dlerror()'s return value.


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